Since verticals are themselves a stack and you're going to want a flu damper anyway - I would just put a short peice of 4" pipe on for a stack and a good stack cutoff plate to control the opening completely down to zero - you should be good.
And on the eighth day God created barbecue …. because he DOES love us and he wants us to be happy.
Current smokers: Egor (trailered RF) and Easybake (tabletop pellet drive)
Here's the firebox I made to slide into the bottom. The size is 18 x 30 x 17. The top of the box (not shown) has a 7 x 7 opening with a diverted plate 3" above the opening. I was thinking 2" for an intake. Does that sound about right?